1. Field of the Invention:
The invention is directed to an ash treatment system and process for use in the fluidized bed combustion of waste fuels having a high ash content.
2. Description:
Fluidized bed reactors are well-known means for generating heat and, in various forms, carry out processes such as drying, roasting, calcining, incineration and heat treatment of solids with gases in the chemical, metallurgical and other material processing fields. In the form of fluid bed boilers, steam is generated for use in driving electric power generation equipment, for process heat, for space heating, or for other purposes.
Fluidized bed reactors typically comprise a vessel in which a bed of particulate solids is present in the reaction chamber. Sufficient air or other gas is introduced into the vessel below the bed of particulate solids in a volume sufficient to achieve a gas velocity that expands or fluidizes the solids bed, suspending the particulate solids of the bed in the flowing air stream and imparting to the individual particles a continuous random motion with the fluidized bed as a whole resembling a boiling liquid. Conducting a combustion reaction in a fluidized bed has important advantages which include attainment of a substantially uniform bed temperature, combustion at relatively low temperatures and a high heat transfer rate.
Combustion of solid fuels such as coal in a fluid bed reactor involves the gasification of the organic component of the fuel leaving a residue of solid ash particles. When burning waste fuels of high ash content in a fluidized bed, the need to continuously remove from the combusting fluidized bed the relatively large quantities of red hot ash becomes a serious problem. In the reactor the very finest ash particles will be elutriated by the gases flowing in the reactor and will exit through the stack with the exhaust gases. Ash particles of somewhat larger particle sizes will become part of the fluidized bed where they improve the operation of the fluidized bed by retaining heat and contacting and igniting fresh fuel particles. The continuous motion of the ash particles in that fluidized bed brings about numerous collisions between ash particles in a softened condition due to the elevated temperature. Under such conditions, ash agglomerates readily form and these agglomerates grow to a size such that they are no longer fluidizable and they tend to descend toward the bottom of the fluidized bed coming to rest upon the air distribution plate located beneath the fluidized bed. Such an accumulation of large ash particles and large ash agglomerates on the air distribution plate will ultimately cause defluidization of the fluidized bed and subsequent shutdown.
Accordingly, it is well recognized that the accumulation of excess coarse ash particles and oversized ash agglomerates must be removed from the fluidized bed. As the coarse ash particles are removed from the bed, it is unavoidable that a substantial amount of ash fines are also removed. The ash removed is at a relatively high temperature and represents a heat loss, if steps are not taken to recover the heat. In addition, the ash particles removed from the fluidized bed invariably have associated with them a significant component of unburned carbon. The unburned carbon represents a loss of combustion efficiency and it would represent a much sought-after improvement if this carbon could be usefully burned to enhance reactor operation.
To exemplify the problem, a waste coal or anthracite may consist of two-thirds ash much of which is in the form of stone or rock and therefore tends to stay substantially in the same size range as the feed material to the fluidized bed boiler. A conventional cooler may be attached to the ash duct from the combustor with the ash cooled in a stream of cold air which also strips out the fines for return to the combustion compartment with the air. Such a unit is known as a classifier. Alternatively, the ash may be directed into a second fluidized bed and simply cooled with air or additional water-cooled tubes in the bed to remove the heat. Such a unit is a fluidized bed cooler. A third possibility is to simply have a water-cooled screw transporting the ash and removing the heat. These known devices have the disadvantage that they have only one function, cooling, or at most two, classifying and cooling at the same time.
As another consideration, fluidized bed boilers operating on waste fuels have to build up to a high carbon level in the combusting fluidized bed in order to achieve the proper combustion temperature which is typically about 1600.degree. F. It will be understood that withdrawing the ash from the fluidized bed reactor not only removes heat from the reactor, but also removes unburned carbon which in the classifier or fluidized bed cooler largely goes to waste. A small amount of the carbon may be burned because of the air present, but the rapidly quenching nature of the cooler or classifier means that the reaction rate is not maintained and significant unburned carbon is ejected from the system in the ash. This, of course, negatively impacts on overall boiler and system efficiency.
Some of the related prior art is indicated below with comment on the disclosed subject matter.
U.S. Pat. No. 4,700,636, issued Oct. 20, 1987 discloses an ash classifier device for returning ash fines to a fluidized bed reactor while collecting coarse ash particles for disposal. Only minor cooling of the ash particles is effected.
U.S. Pat. No. 4,598,653, issued July 8, 1986, discloses a combustion system in which fine particles are separated from coarse particles in a gas stream with entrained fine particles combusted in an upper combustor and coarse particles combusted in a lower compartment which may be a bubbling fluid bed combustor. There is provision for returning uncombusted particles to the upper or lower compartment.
U.S. Pat. No. 4,330,502, issued May 18, 1982, discloses a modified fluidized bed reactor having an ash classification system for separating and returning fines to the reactor while discharging coarse particles from the reactor.
U.S. Pat. No. 4,301,771, issued Nov. 24, 1981, discloses a fluidized bed reactor with internal structure for separating fines from the combustion gases and returning them to the fluidized bed.
U.S. Pat. No. 3,397,657, issued Aug. 20, 1968, discloses a fluidized bed reactor wherein non-inflammable materials are separated and discharged from the system while the fluidized medium (fines) are returned to the reactor.
U.S. Pat. No. 3,001,228, issued Sept. 26, 1961, discloses a fluidized bed system for coating and pelletizing fusible materials. The process involves coating molten droplets with solids in an upper fluidized bed and collecting the coated pellets in a lower fluidized bed. Excess particles are removed from the lower fluidized bed to a fluidized bed maintained in an excess particle compartment.